This invention relates to controlling the thickness of a hydrous oxide film on aluminum capacitor foil by producing the film in a dilute borate solution prior to the anodization of the foil.
It is known to produce hydrous oxide films on aluminum electrolytic capacitor foil by contacting the foil with hot or boiling water. The thickness of the film depends mainly on the contacting time. Because the initial rate of reaction is rapid, it has been difficult to control film thickness when thin films are required. One way of controlling such thickness has been to decrease reaction time to less than one minute. Another method of controlling thickness has been to carry the reaction out below 100.degree. C., e.g., 85.degree. C.
The prior art processes have been satisfactory for most foils, but have not always been as reliable as desired when etched foil for low-voltage capacitors is involved. For example, it is commonly known that hydrous oxide growth does not occur instantaneously upon immersion in hot water but that a brief induction period occurs before reaction starts. This period can vary by a few seconds, depending upon foil condition, and therefore the actual reaction time--and then the film thickness--can be seriously affected when the total immersion time is below one minute.
It is also known that small amounts of impurities in hard water suppress the growth of the hydrous oxide layer. Such suppression takes place when silicates, sulfates, carbonates, citrates, borates, oxalates, phosphates, and chromates are present. When small amounts of these materials were used, the layers were of normal thickness but unstable; when larger amounts were used, the layers were thin and gave little protection against water or corrosive materials.